This application seeks support for a project of research directed toward better understanding of the molecular mechanisms and cellular functions of chloride and phosphate transport across human red blood cell membranes. Our investigation of the molecular mechanism of anion transport seeks to solve four major unsolved problems: First, the role of membrane lipid surface charge and membrane proteins and protein fragments on the kinetic asymmetry of anion transport; second, the mechanism of phosphate transport including a determination of the anionic phosphate species that is transported; third, the factors involved in the incorporation of functional membrane proteins into artifical lipid bilayers; fourth, the electrical properties of the anion exchange mechanisms and anion conductance. Experiments designed to answer questions relevant to these problems involve measurements under various conditions of tracer and net fluxes of chloride, phosphate, sodium and other small ions across human red blood cells in vitro, and artificial thin lipid bilayers; measurements of electrical properties of thin lipid membranes; and measurements of membrane reconstruction from vesicles and monolayer films. The results of this project will be of importance to biologists, physiologists, biophysicists and biochemists and others concerned with basic structure and function of membranes. They also will be important to respiratory physiologists and clinicians concerned with the rate limiting steps in carbon dioxide excretion and to renal physiologists and clinicians concerned with mechanisms of diuretics and their toxicity to patients.